I explored the North East part of this park. In some ways it's a similar open space to Hackney Marshes. Loads and loads of football pitches! And wild space around the edges. A couple of major differences though: Hackney wins on the presence of the river and canal along the edges (though it's true the River Lea isn't too fragrant right now.). On the other hand Wanstead seems better used, with many of the pitches in noisy use when I passed. There were also larger areas of serious woods.
There's some hints of a grander past at Wanstead. Though where I entered the park is bordered by some very ugly blocks of flats, further north, away from the pitches, there's the remains of a grand horse ride, and some rather grand houses along the edge.
If you want a manicured park, then this isn't for you. Personally I'm looking forward to exploring more. Maybe I'll make it down to the parts that extend towards Aldersbrook Road next time I go read more